Rotary screen coaters are known for applying a hot melt material, such as an adhesive, to a moving web, such as raw paper or label stock, passing by the coater. Typically, these rotary screen coaters include a die located within and fixed in position with respect to a rotating cylindrical screen. The die applies a supply of adhesive to an inner surface of the cylindrical screen and forces it through openings in the screen to the moving web. Examples of such devices include the apparatus shown in U.S. Pat. No. 5,213,033 to Bourgeois et al., and the Nordson rotary screen coater, manufactured by the assignee of the present invention.
Operational difficulties may be encountered with such rotary screen coaters if the adhesive is not applied to the inner surface of the screen in a continuous, even, and homogeneous manner. For example, if an excess of adhesive is provided at the die mouth opening near the ends of the cylindrical screen, or if adhesive is permitted to accumulate over time at these end locations to create such an excess, the mechanism for driving the screen may become jammed.
Voids such as air pockets in the supply of adhesive at the inner surface of the screen also pose a problem if the voids are permitted to pass through the screen onto the surface of the web to be coated. Such voids may be initially present at the mouth of the die if the die does not provide a continuous supply of adhesive. Voids may also develop outside of the mouth of the die near the inner surface of the screen as the adhesive supply at this location rotates along with the screen, thereby becoming impregnated with air pockets.
Still further problems may be encountered if the adhesive, the liquidity of which is usually maintained by heating the die body, is permitted to cool between the time when it exits the die mouth opening and the time when it is forced through the openings in the screen, typically by a wiper extending from the die opening. Adhesive which cools sufficiently prior to its passage through the screen may clog the screen openings. In addition, particulate matter which passes through the die mouth opening may also disrupt the flow of adhesive through these openings.
Versatility is also important in the design of the die. Because varying thicknesses of adhesive coatings are used for different types of webs to be coated, and because the webs are often of varying widths, the die should accommodate a variety of anticipated coating operations. Therefore, it is desirable that both the thickness of the die mouth opening and the effective coating width of the mouth be adjustable.
Accordingly, it is an object of the present invention to provide a die for use in a rotary screen coater which provides for a continuous, even, and homogeneous distribution of adhesive along a mouth opening extending the width thereof. It is an additional object of the invention to provide such a die which facilitates adjustment of both the effective width of the mouth opening to accommodate webs of differing widths and the thickness of the mouth opening.